This invention relates to a wireless base station multiple-beam antenna system in DS-CDMA wireless communications. More particularly, the invention relates to a multiple-beam antenna system having an uplink beam forming function for performing reception upon applying uplink beam forming to signals that have been received by a plurality of antenna elements, and a downlink beam forming function for applying downlink beam forming to transmission signals in order to form a transmission beam in a prescribed direction.
Digital cellular wireless communication systems using DS-CDMA (Direct Sequence Code Division Multiple Access) technology have been developed as next-generation mobile communications systems for implementing wireless multimedia communication. In a communications system using DS-CDMA, interference between users is the main cause of a decline in channel capacity and transmission quality of cells. Research and development in regard to multiple-beam antennas and adaptive array antennas is being carried out in an effort to discover techniques for reducing such interference and improving transmission quality.
As shown in FIG. 10, a multiple-beam antenna performs reception using an array antenna AAT consisting of a plurality of element antennas AT1-ATN, and applies beam forming to antenna output signals by means of a beam former BMF to electrically form multiple beams B1-BM of prescribed directivity. Each beam of the multiple-beam antenna possesses a directivity pattern of the kind shown in FIG. 11. Accordingly, radio waves emitted from an ith user (mobile station) residing in the directivity direction of beam 2, for example, are received by the array antenna AAT and the beam former BMF outputs the beams B1-BM. The power of beam B2, however, is greater than that of the other beams B1, B3-BM. Data is subsequently demodulated by performing despreading using the beam B2. Thus, in accordance with a multiple-beam antenna, reception is performed upon selecting the optimum beam on a per-user (channel) basis, whereby there are obtained such effects as a reduction in interference between channels, an improvement in reception SN ratio owing to a higher antenna gain and a reduction in terminal transmission power.
The foregoing relates to a reception beam former which electrically forms the plurality of uplink reception beams B1-BM by applying uplink reception beam forming to the signals received by the plurality of antenna elements AT1-ATN of the array antenna AAT. However, a transmission beam former can be provided as well. More specifically, it is possible to provide a transmission beam former in such a manner that downlink transmission beam forming is applied to transmission signals to generate antenna element input signals and the signals are input to individual antenna elements, whereby beams having directivities in prescribed directions are output from the antenna.
FIG. 12 is a diagram showing the construction of a wireless base station using a transceiving multiple-beam antenna. An array antenna AAT1 on the receiving side has a plurality of antenna elements ATR1-ATRN. An array antenna AAT2 on the transmitting side has a plurality of antenna elements ATT1-ATTN. A reception beam former RBF electrically forms M-number of uplink reception beams B1-BM by applying uplink reception beam forming to signals that have been received by the N-number of antenna elements ATR1-ATRN. Channel receivers CHR1-CHRK are provided for respective channels (users or mobile units) and are equipped with despreading circuits, synchronous detection circuits and data identification circuits, etc. The output signals of the reception beam former RBF enter each of the channel receivers CHR1-CHRK. Channel transmitters CHT1-CHTK are provided for respective channels and are equipped with spreading circuits and quadrature modulation circuits, etc. Transmission data on respective channels enter respective ones of the channel transmitters CHT1-CHTK. A transmission beam former TBF generates antenna element input signals by applying downlink transmission beam forming to transmission signals (transmission beams) output from the channel transmitters.
As illustrated in FIG. 13, the reception beam former RBF multiplies output signals x1-xN of the respective antenna elements by weights Wk,i to thereby implement phase rotation, and sums the products to electrically form M-number of uplink reception beams 1xe2x88x92M each having a prescribed directivity. If xk(nTc) represents the reception signals of N-number of antenna elements and Wk,i represents a conversion coefficient of the beam former, then a signal yi(nTc) of an ith beam (i=1xe2x88x92M) will be expressed by the following:
yi(nTc)=xcexa3Wk,ixc2x7xk(nTc)(k=1xe2x88x92N)xe2x80x83xe2x80x83(1)
The direction (directivity) of each of the M beams can be applied to the array antenna by deciding the conversion coefficient Wk,i. As a result, a transmission signal from a user (mobile station) in a prescribed ith directivity direction can be obtained from a terminal, e.g., the ith terminal, that corresponds to the ith directivity direction of the reception beam former RBF.
As illustrated in FIG. 14, the transmission beam former TBF splits a transmission signal (transmission beam) yi that enters an ith input terminal into N branches and multiplies each branch signal Yi by the weight Wk,i (k=1xe2x88x92N) to implement phase rotation and generate a signal xk (k=1xe2x88x92N) that is input to a respective one of the N transmitting antennas. In this case, xk is represented by the following:
xe2x80x83xk=Wk,ixc2x7yixe2x80x83xe2x80x83(2)
The direction (directivity) of each of the M beams can be applied to the array antenna by deciding the conversion coefficient Wk,i. As a result, if it is desired to make a transmission to a user (mobile station) in an ith transmission beam direction, the transmission signal yi should be input to the ith input terminal of the transmission beam former TBF.
Thus, multiple beams produced by the reception beam former RBF and multiple beams produced by the transmission beam former TBF are made to coincide. Consequently, in order to communicate with a user (mobile station) in the ith beam direction, it will suffice to despread the beam output by the ith output terminal of the reception beam former RBF and demodulate the data. In order to transmit data, it will suffice to input the transmission signal to the ith input terminal of the transmission beam former TBF. More specifically, reception signals xi(nTc) (i=1xe2x88x92N) from N-number of antenna elements ATR1-ATRN are amplified, detected and subjected to an A/D conversion by means that are not shown. The reception beam former RBF then digitally forms M-number of beams. That is, the reception beam former RBF obtains the signal yi(nTc) of each beam through the conversion expressed by Equation (1). Next, the reception beam former RBF performs despreading on a per-channel basis in regard to the plurality of beams formed and carries out uplink reception upon selecting the beam for which signal power after despreading is largest or the beam for which the correlation power between a pilot signal after despreading and a reference signal is largest. In case of downlink transmission, a transmission signal is input to the ith input terminal of the transmission beam former TBF in such a manner that the direction obtained will be the same as that of the beam that was selected at the time of uplink reception. As a result, the transmission array antenna AAT2 radiates the transmission signal toward the user (mobile station) in the ith beam direction.
FIG. 15 shows another example of a beam former. This is a diagram showing the construction of the well-known Butler matrix (in the case of an 8-beam antenna). FIG. 16 is a diagram useful in describing multiple beams formed by the Butler matrix.
The transmission beam former TBF in FIG. 15 is obtained by combining hybrid circuits, each of which has two input terminals and two output terminals, and phase shifters for delaying phase by a predetermined amount. Input terminals 1R-4R, 1L-4L are connected to all radiating elements ATT1-ATT8 (#1-#8). This transmission beam former TBF includes hybrid circuits HYB the output terminals A and B of which provide equal power, with the phase of output terminal B lagging that of output terminal A by xcfx80/2 (=90xc2x0). The encircled numerals indicate the phase shifters; if the numeral is m, then the phase shift is mxcfx80/8. For example, if a signal enters a hybrid circuit HYB from the terminal 1R, the output terminals A, B deliver equal power but the phase is delayed by 90xc2x0 (xcfx80/2) at terminal B.
In FIG. 15, amounts of phase shift at the #1-#8 antenna elements are calculated with respect to an input signal from the input terminal 1R. The amount of phase shift of a connection cable, however, may be considered negligible. Phase is 5xcfx80/8 at antenna element #1, 6xcfx80/8 at antenna element #2, 7xcfx80/8 at antenna element #3, 8xcfx80/8 at antenna element #4, 9xcfx80/8 at antenna element #5, 10xcfx80/8 at antenna element #6, 11xcfx80/8 at antenna element #7 and 12xcfx80/8 at antenna element #8. Feed is carried out with phase being delayed in increments of xcfx80/8 from antenna element #1 to antenna element #8.
When the position of the terminal to which the input signal is fed changes in FIG. 15, the phase difference that develops between the radiating elements grows larger and a beam is formed in a direction that is significantly offset from the front side of the array. If we let xcex94xcexa8 represent the phase difference between radiating elements and d the element spacing, then beam direction xcex8 will be expressed as follows:
dxc2x7sin xcex8/xcex=xcex94xcexa8/2xcfx80xe2x80x83xe2x80x83(3)
In case of an input from the aforementioned input terminal 1R, we would have sin xcex8=1/8, or xcex8≈7.2xc2x0, if xcex94xcexa8=xcfx80/8 and d=xcex/2 hold. The result is that a beam is formed in the direction of 1R in FIG. 16. Further, since xcex94xcexa8=7xcfx80/8 holds in regard to an input from the input terminal 4R, in this case we have sin xcex8=7/8, xcex8≈61xc2x0.
In mobile communications, there is not only a communication mode in which information is transmitted continuously, as in the case of voice communication, but also a communication mode in which transmission is bursty, as in the communication of data in the form of packets. When there is no information to be transmitted in packet communication, the usual practice is to transmit nothing in order to reduce interference with respect to other stations. In a wireless base station, there are cases where there is an uplink reception signal but no downlink transmission signal with regard to a certain channel (mobile station). Since the wireless base station does not receive an uplink signal from the mobile station in such cases, in which beam area the mobile station is currently situated is unknown and it is not possible to decide the directivity direction of the downlink transmission beam. In other words, downlink beam forming cannot be carried out in such cases. Accordingly, the conventional practice is to perform beam forming only for uplink reception and not for downlink transmission taking into account the mode in which data is transmitted in bursts.
FIG. 17 is a block diagram showing the construction (in the case of four beams) of a prior-art channel transceiver which performs only uplink beam forming. The apparatus includes the reception array antenna AAT1, which has antenna elements ATR1-ATR4, the reception beam former RBF, the transmitting antenna AAT2, and a transceiver channel unit RTi of an ith channel having a channel receiver CHRi of the ith channel and a channel transmitter CHTi of the ith channel. Though not illustrated, receiver circuitry for performing a frequency conversion, detection and an A/D conversion is provided in front of the reception beam former RBF in the receiving system. Further, though not illustrated, a transmission unit for performing a D/A conversion, frequency conversion and amplification is provided in front of the transmitting antenna in the transmission system.
The channel receiver CHRi includes despreading circuits 11-14 for applying despread processing to beams 1-4, which are output by the reception beam former RBF, using a spreading code that has been allocated to the channel; a selector 2 for selecting the optimum beam (despread signal); a selection controller 3 for deciding the beam for which signal power is maximum or the beam for which the cross-correlation power between a reception pilot signal and a reference signal is maximum, and for so notifying the selector 2; a synchronous detector 4 for subjecting the despread signal selected by the selector 2 to synchronous detection; an error corrector 5 for performing an error correction using an error correction code appended on the transmitting side; and a data identification unit 6 for identifying received data. The channel transmitter CHTi includes an error correction coder 7 for adding an error correction code onto transmission data, a modulator 8 such as a QPSK quadrature modulator, and a spread-spectrum modulator 9 for spreading and outputting transmission data using the spreading code allocated to the mobile station (channel).
In accordance with this transceiver channel unit, beam forming is performed only for uplink reception and not for downlink transmission.
Thus, with the conventional wireless base station, transmission beam forming is not applied even in an ordinary communication mode in which transmission is not bursty. Consequently, downlink beam forming is not applied also in a case where an uplink signal is present. As a result, interference between channels at the time of downlink communication cannot be reduced, reception SN ratio cannot be improved and it is not possible to reduce terminal transmission power.
Accordingly, an object of the present invention is to so arrange it that downlink beam forming can be carried out in a case where an uplink reception signal exists.
Another object of the present invention is to so arrange it that downlink beam forming at a base station can be performed at all times regardless of whether there is uplink transmission information.
In accordance with the present invention, the foregoing objects are attained by providing a multiple-beam antenna system of a wireless base station in CDMA mobile communications comprising (1) a reception beam former for electrically forming a plurality of uplink reception beams by applying uplink beam forming to signals received by a plurality of antenna elements of an antenna array; (2) a reception data identification unit for executing reception data identification processing based upon an optimum beam among the plurality of uplink reception beams; (3) a transmission beam former for generating antenna element input signals by applying downlink beam forming, which is for beam formation in a prescribed direction, to transmission signals; and (4) means for controlling whether downlink beam forming, which is for forming a downlink transmission beam in a direction identical with that of the optimum uplink reception beam, is performed or not.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.